Observer-Based Disturbance Suppression in a 3-DoF Positioning System for High-Precision 3D Inline Metrology

This paper proposes an advanced disturbance-suppression strategy for a dual-stage actuated 3-degree-of-freedom (3-DoF) positioning system for high-precise optical 3D inline metrology applications. The decentralized control loop based on single input single output (SISO) PID controllers is complemented by a disturbance observer (DOB) in each DoF to suppress disturbance forces, originating from the coarse positioning unit at fractions of the pitch of the mechanical spindle drive. By reformulating the DOB design problem in the H∞-synthesis framework, the disturbance sensitivity of the closed-loop system is shaped to suppress disturbances at these specific frequencies. Experiments on an experimental prototype demonstrate the effectiveness of the approach with up to 28% reduction of measurement errors related to the positioning system on a large inspection area <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(0.7 \times 0.6m)$</tex>.